The present invention relates to new improved petroleum resins and their production. In particular, the invention relates to petroleum resins suitable as tackifiers for block copolymer based hot melt adhesives and pressure sensitive adhesives, and ethylene-unsaturated ester copolymer based hot melt adhesives. The present invention also relates to a new process for hydrogenating petroleum resins.
Adhesives play an important role in everyone's daily activities and are used in many different forms such as tape, means to close packages or secure items, bandages, envelopes, notepads, diaper tabs, or any one of many other products in common usage. The key performance requirements for adhesives are that they should have suitable cohesive and adhesive properties at end use conditions and during application to substrates while being aesthetically pleasing to consumers. Usually these adhesives are prepared from a mixture of tackifier resins and polymers.
Block copolymers made from styrene and a diolefin are widely used in the production of adhesives. Typical block copolymers are styrene/isoprene/styrene (SIS), styrene/butadiene/styrene (SBS), hydrogenated styrene/butadiene/styrene (SEBS) and the like. While various block copolymers may be used, SBS and SIS copolymers are found to be particularly useful in the present invention. These copolymers are typically blended with tackifying resins and oil in order to form adhesives.
Ethylene-unsaturated ester copolymers are also widely used in the adhesives industry. These copolymers include the group consisting of ethylene copolymerized with an unsaturated ester which contains both an ester group and a carbon-carbon double bond. While various ethylene-unsaturated ester copolymers may be used, ethylene-vinyl acetate copolymers are found to be particularly useful.
The use of SBS block copolymers has proved particularly troublesome to the Adhesives Industry in terms of identifying a suitable tackifier resin. It has been known in preparing adhesives using SIS copolymers that aliphatic tackifier resins may be used with success. Unfortunately, the aliphatic resins, while meeting the requirements for lower color adhesives, do not have adequate compatibility with SBS copolymers to form an acceptable adhesive.
It has been found that the desired adhesive properties are not present with aliphatic tackifier resins in SBS systems because of the higher midblock solubility parameter component of SBS copolymers versus SIS copolymers. While aliphatic tackifier resins, with relatively low solubility parameters, are compatible with the midblock of SIS copolymers, they exhibit limited compatibility with the higher solubility parameter midblock of SBS copolymers. This problem is also compounded by the use of oil in many formulations. If a higher aromatic level tackifier resin is used, a more compatible blend is formed with the SBS midblock, particularly in the presence of oils.
The introduction of aromaticity into the tackifier resin also results in a softer adhesive system which is advantageous in many uses. This is due to the interaction of the resin with the hard styrene domains of SBS or SIS, which results in a reduced plateau modulus. This principle is particularly important for block copolymers, either SBS or SIS, which have relatively high levels of styrene content and are thus relatively hard.
Attempts to make use of the positive compatibility characteristics associated with the introduction of higher levels of aromatic compounds into tackifier resins have generally caused an unacceptably high color level for usage in many modern adhesives (British Patent 1,377,989). Previous efforts to overcome the difficulties associated with introduction of aromatic compounds in tackifier resins have been directed towards polymerizing mixtures of aliphatic and aromatic compounds. British Patent 1,447,419 describes a hot melt pressure sensitive adhesive composition based on block polymers, including SBS, and a tackifier obtained by catalytically polymerizing and hydrogenating a mixture of aromatic and aliphatic compounds. However, problems arise from the use of such mixtures. While aromatic and aliphatic compounds are easily polymerized in the presence of a metal halide catalyst such as anhydrous aluminum chloride, the use of such catalyst has been found to have deleterious effects in the production of tackifier resins.
Because of the high color characteristics associated with the introduction of aromatic compounds into tackifier resins, the resins must be hydrogenated to reduce their color values in order to be used in many adhesive systems. The use of metal halide catalysts in the polymerization step leads to the incorporation of halide groups in the resin. These halide groups are very difficult to remove and if not removed from the resin, prior to the necessary hydrogenation process, will become bound to the hydrogenation catalyst during hydrogenation. The effect of halides is to "poison" the catalyst which results in decreased hydrogenation activity and increased cracking of the tackifier resin. The results of such catalyst "poisoning" may include poor color and stability of the hydrogenated resin as well as a degraded molecular weight.
EP-A-260001 discloses a hydrogenated resin made by catalytic polymerization and hydrogenation of a mixture of aromatic, aliphatic and terpene compounds. Examples show the use of such resins in SBS block copolymer based adhesives. The terpene component does not originate in the cracking of petroleum feed streams and is more costly. EP-A-149909 discloses a resin for pressure sensitive adhesive compositions containing hydrogenated butadiene/styrene block copolymer. A feed containing cyclopentadiene and derivatives is thermally polymerized and hydrogenated. The application states that the feed may contain aromatics.
Accordingly, it is the object of the present invention to provide halide free, low color petroleum resins, thermally polymerized from a feed which is comprised of a vinyl aromatic component, a cyclodiene component and optionally an acyclic diene component. It is a further object of the present invention to provide a novel hydrogenation process to obtain low color levels of thermally polymerized petroleum resins. It is a further object of the present invention to provide novel block copolymer based hot melt and pressure sensitive adhesive systems. Yet a further object of the present invention is to provide hot melt adhesive systems based on ethylene-unsaturated ester copolymers.